Although a substantial amount of research has been conducted on the determinants of breast cancer and environmental influences on cancer generally, there has been remarkably little effort to address environmental determinants of breast cancer. Starting with successful, independent research programs in breast cancer and environmental epidemiology, we propose to join the two though development of a team of committed investigators and set of interrelated projects. The two environmental agents of greatest potential importance in the etiology of breast cancer based on available theoretical, experimental, and epidemiologic evidence and pesticides, particularly persistent chlorinated hydrocarbon compounds, and electromagnetic fields. In addition, genetic polymorphisms in carcinogen-metabolizing enzymes have been found to modify the effect of environmental exposures on lung and bladder cancer and may modify the effect of pesticides on breast cancer as well. We propose to hold a workshop with a small number of outside experts on electromagnetic fields and breast cancer. We plan a series of two pilot projects on this topic. Conducting these pilot projects over the four- year funding period would provide critical experience and pilot data to design and complete successfully and full-scale epidemiologic studies to address the role electromagnetic fields in breast cancer etiology. A determination of whether these complex agents contribute to the etiology of breast cancer will only be achieved through merging the talents and methods of capable researchers in both environmental and breast cancer epidemiology. These efforts will culminate in a team of collaborating researchers with interests and experience in studying breast cancer and the environment, along with graduate students and postdoctoral fellows who share those interests through the activities proposed in this application, we will develop the infrastructure to address a wide range of potential environmental contributors to breast cancer etiology with the most sophisticated techniques of both environmental and breast cancer epidemiology.